Rare-earth-doped fiber amplifiers have been increasingly deployed in fiber optic signal transmission systems. Previously, over long distance fiber optic transmission links, the optical signal was detected at periodic distances by an opto-electronic detector, converted into an electrical signal, which was then used to drive a laser diode to in effect regenerate the optical signal for retransmission over the next section of the link. The distance between these opto-electronic systems was dictated by the attenuation at the signal frequencies of the fiber, and if any one of these opto-electronic devices failed, the entire optical transmission link failed. With the advent of fiber amplifiers, the distance between electro-optical devices changed from attenuation-limited to dispersion limited, however.
Under the model, the fiber amplifiers are distributed along the link to amplify the signal to counter its attenuation. Opto-electronic devices are only provided along the link at distances beyond which problematic chromatic dispersion and other effects would impair signal demodulation.
Another advantage associated with the use of fiber amplifiers in optical transmission links is related to their broad gain spectrum. This feature makes dense wavelength division multiplexed systems realistic since an amplifier can simultaneously amplify multiple channels traveling through the same optical fiber. Currently, WDM systems using fiber amplifier amplification have been deployed with 50-100 channels, with even larger channel systems being proposed.
The fiber amplifier systems require relatively few components. They comprise a rare-earth-doped fiber. Commonly, erbium-doped fiber amplifiers are used since they have a gain spectrum surrounding 1550 nanometers (nm), where there is a transmission window in commonly deployed fiber optic cabling. The fiber amplifier is anti-reflection coated at both its input and output facets to ensure that it behaves as an amplifier and not a laser, and isolators are also used, typically at the fiber output. Such fiber amplifiers are usually pumped by laser diode pumps, operating at 980 nm or 1480 nm have been used, but 980 nm pumping appears to becoming increasingly the standard due to certain stabilities in the fiber at this frequency and the laser diode availability. Clean signal amplification is further achieved by closely regulating the pump light. Typically, the pumps are tuned for temporal and spectral stability. Fiber grating stabilization is also used in some systems.